Hi there!
No need to pretend bro. There are many.. erm should i say.. dishonest people in the world who will cheat and squander your hard earned money just so they can make a quick profit. So if you don't wanna be fooled you gotta do your research and equip yourself with knowledge. Personally i don't think you have to play some crazy riff to show the buyer that you know your stuff. Whats important is knowing the product inside out.
Firstly you must have the background knowledge of the product, in this case a guitar. Google can make you a guitar expert in less than a day. Most importantly, is to able to tell if the guitar your are buying is genuine or not. This is important when you are buying more high end guitars such as Fender, Gibson, PRS and so on. There are many fakes out there and just by googling you can get a many information on how to spot a fake. But of course the easiest way is to check the serial number or ask for the cert of authencity (if the seller has one).
Next thing is to check the state of the guitar. Both cosmetic and electronics wise. Just take your time to test the guitar. Its your money and its your right to take your time. Some of the points to take note.
-check for fret buzz
you can do this by playing every single fret on the guitar one by one slowly. if the seller says he sent the guitar for setup recently, they shouldn't be any buzz. the action of the guitar should be good as well.
-try all the pickups
use the pickup selector and test the pickup one by one. note the volume and tone for each pickup. the volume should be almost the same for all pickups.
-check the tuning head
try to tune the guitar. usually i will drop d, just to test the tuning heads.
-cosmetic wise
this depends on how sharp your eye is. make sure you are testing the guitar in a well lit room. see if they are any rusts/ dings. theres bound to be if its used. if its new, then it shouldn't be present.
-tone and feel wise
this is entirely subjected to your own preference. Best is to try many guitars before purchasing the guitar that you want.
-check the neck
see if its straight. a warped neck is the last thing you want from a guitar.
All this testing can be done by playing simple chords or simple licks/bends. Nothing crazy is needed. Whats important is to hear the sound and tone that is produced. Unless you wanna show off then okay laa. (personally im turned off by those ppl whu show off when testing). haa! but if you are good then no harm showing off.
It is also important to make conversation with the seller. Just talk normally, and of course confidently. You can talk about gear, music anything actually to build rapport. We are after all softies. Then you can ask him more about the product you are buying. For example where did he buy from, how much did he buy, any defects present. If you are nice enuff, he might even give you a discount or a freebie. At the end of the day, it would be best if you are satisfied with your purchase and even get a new friend. =)
All the best for your guitar quest!